A solid electrolytic capacitor is constituted by an electrical conductor (anode body) as one electrode, a dielectric layer formed on the surface layer of the electrode, and the other electrode (semiconductor layer) provided thereon. In the manufacture of a solid electrolytic capacitor comprising a semiconductor layer made of conductive polymer, forming of the dielectric layer on the anode body by chemical conversion and forming of the semiconductor layer thereon by electropolymerization are sequentially performed. Generally, a plurality of anode bodies are electrically connected in parallel, and such treatment as anodic oxidation (chemical conversion treatment) and electropolymerization (formation of a semiconductor layer) is carried out on the anode bodies at one time.
In the case of forming a semiconductor layer on a plurality of anode bodies, the following problem occurs because respective anode bodies are not always homogeneous and also the semiconductor formation rate may vary among the anode bodies. That is, the value of current flowing through each of the anode bodies cannot be constant, and in an extreme case, one anode body becomes defective (short-circuited state), and the current concentrates on the anode body while almost no current flows through the other anode bodies. In light of the above, the inventors of the present invention have proposed the method of forming a semiconductor layer by subjecting chemically-converted anode bodies to constant current electropolymerization by means of circuits including constant current sources (an internal jig for electrolytic polymerization) (Patent Document 1; JP 2005-244154 A (US 2007/101565 A1).
As a method for producing an electrolytic capacitor element which can obtain a group of capacitor elements having a narrow capacitance distribution and a low ESR by reducing the time for forming a dielectric layer (chemical conversion treatment), which requires time in the process for producing a capacitor, and by selecting an optimum amount of current depending on the stage of chemical conversion treatment and electropolymerization, the present inventors have filed a patent application relating to a method for manufacturing an electrolytic capacitor element including the steps of: forming a dielectric layer on surfaces of a plurality of anode bodies by anodic oxidation at one time; and forming a semiconductor layer on the dielectric layer, in which the anodic oxidation is performed by limiting a current for the anodic oxidation with respect to the individual anode bodies (Patent Document: 2 WO 2010/107011 (US 2012/014036 1)).
It is necessary in some cases to impart heat resistance to the dielectric layer by leaving a produced capacitor at a high temperature exceeding 200° C. after forming a dielectric layer, cooling it to room temperature, and subjecting it again to the chemical conversion treatment in order to improve the heat resistance of the produced capacitors (particularly, the stability of the leakage current (LC) value when the capacitor is subjected to a high-temperature treatment). Since the heatproof temperature of the electronic parts to be mounted on the jig for electrolytic polymerization as disclosed by Patent Document 1 is generally 150° C. or lower, it is not possible to leave the jig to which conductors (such as a sintered body and a valve-acting metal foil) are joined at a high temperature exceeding 200° C. Therefore, it might be possible to prepare a jig to be left at a high temperature and a jig for internal electrolytic polymerization separately and to use the jigs by switching the conductors from one jig to another. However, it is very difficult to reconnect a number of conductors having a dielectric layer formed thereon to a jig with a proper connecting distance without damaging the formed dielectric layer, and is not practical.
JP-A-H02-298010 (Patent Document 3) discloses a method for electrolytic polymerization designed to connect a constant current element (constant current diode) to a stainless-steel electrode so as to bring the constant current diode into electric contact with the metal oxide layer (semiconductor layer) formed on the surface of an anode body. However, when one attempts to form a semiconductor layer by performing electrolytic polymerization with constant current within a practical period of time by this method and when the anode body is a sintered body having a high CV value, it is difficult to attain a high impregnation rate (80% or higher). Also, when the sintered body is large in volume (20 mm3 or more), it is difficult to attain a high impregnation rate even if a sintered body has a low CV value. Meanwhile, performing electrolytic polymerization at a slow pace at a low constant current cannot be employed in the industrial production since it reduces the productivity.
Generally, it is expected that the capacitance change ratio of a solid electrolytic capacitor in the humidity resistance test is within ±20%. When the impregnation of a solid electrolytic capacitor is less than 80%, there is a possibility that 20% or more of the dielectric layer on which a semiconductor (cathode) is not attached exists inside the pores of the anode body. Depending on the environment in which the capacitor is placed, the part of the dielectric layer on which a semiconductor layer is not attached is subject to humidity and serves as a temporary cathode, which increases the capacitance. As a result, the capacitor fails to meet the standard in the humidity resistance test. Also, when moisture gains entry to the part of the dielectric layer on which a semiconductor layer is not attached in the pores of the anode body, it increases the likelihood of the deterioration of the dielectric layer caused by corrosion. For these reasons, it is desirable that the impregnation rate of the semiconductor layer is as high as possible.